Angle-based speed estimation of alternating current machines utilizing a median filter

Abstract

A computer-implemented method for estimating a rotor speed of an alternating current (AC) machine is provided. The method includes determining a stator flux signal based on signals of voltage and current inputs to the AC machine, and determining a rotor flux signal of the AC machine based on the determined stator flux signal. The method includes determining an electrical angle signal based on the determined rotor flux signal, and deriving an electrical frequency signal from the determined electrical angle signal. Subsequently, the method includes sampling the derived electrical frequency signal at a predetermined sampling rate, and storing a predetermined number of sample values. The method further includes evaluating a median value of the predetermined set of electrical frequency sample values, determining a slip frequency value of the AC machine, and determining the rotor speed of the AC machine by subtracting the slip frequency value from the electrical frequency median value.

What is claimed is: 
     
       1. A computer-implemented method for estimating a rotor speed of an alternating current (AC) machine, comprising:
 determining a stator flux signal based on signals of voltage and current inputs to the AC machine; 
 determining a rotor flux signal of the AC machine based on the determined stator flux signal; 
 determining an electrical angle signal based on the determined rotor flux signal; 
 deriving, with a processor, an electrical frequency signal from the determined electrical angle signal; 
 sampling, with the processor, the derived electrical frequency signal at a predetermined sampling rate, and storing a predetermined number of electrical frequency sample values; 
 generating, with a median filter, a median value of a sequence of the predetermined number of electrical frequency sample values by eliminating spikes in the electrical frequency sample values without altering step changes, wherein the median filter is configured to eliminate spikes in an angular derivative signal when the electrical angle signal is reset each cycle; 
 evaluating, with the processor, the median value of the predetermined number of electrical frequency sample values; 
 constantly monitoring stator current commands to the AC machine to determine a slip frequency value of the AC machine; 
 outputting, with the processor, the rotor speed of the AC machine by subtracting the determined slip frequency value from the evaluated electrical frequency median value; and 
 driving the AC machine based on the outputted rotor speed. 
 
     
     
       2. The computer-implemented method of  claim 1 , wherein determining a rotor flux signal of the AC machine based on the determined stator flux signal comprises:
 evaluating a difference between a result of a product of a stator resistance value with one of the current input signals and one of the voltage input signals to produce a difference signal; 
 performing an integration of the produced difference signal, wherein the integration involves a discrete integrator having a sampling rate and a time delay; 
 filtering using a high-pass filtering unit the integrated difference signal in order to minimize a potential accumulated offset value of the integrated difference signal; 
 filtering the high-pass filtered difference using a low-pass filtering unit to extract a residual offset value from the high-pass filtered difference signal; 
 reducing the high-pass filtered difference signal by subtracting the extracted residual offset value from the high-pass filtered difference signal; and 
 determining the rotor flux signal based on the reduced high-pass filtered difference signal using a rotor flux derivation unit. 
 
     
     
       3. The computer-implemented method of  claim 1 , wherein evaluating the median value of the predetermined set of electrical frequency sample values comprises:
 removing data spikes that are generated by a periodic resetting of the electrical angle signal when varying between −π radians and +π radians. 
 
     
     
       4. The computer-implemented method of  claim 1 , wherein evaluating the median value of the predetermined set of electrical frequency sample values comprises:
 using the median filter involves a moving window of N digital samples and corresponding discrete delays. 
 
     
     
       5. The computer-implemented method of  claim 4 , wherein an additional low pass filtering unit is configured to filter output signals of the median filter in order to reduce noise signals, such as ripple noise signals. 
     
     
       6. The computer-implemented method of  claim 4 , wherein signals of voltage and current inputs to the AC machine are provided by a three-phase inverting unit, which is in turn coupled to a current controlling unit that monitors the signals of current inputs to the AC machine. 
     
     
       7. The computer-implemented method of  claim 6 , wherein inputs to the three-phase inverting unit are determined based on the determined electrical angle signal and command input signals provided via a field-oriented control unit. 
     
     
       8. The computer-implemented method of  claim 7 , wherein the command input signals include rotor speed and flux command signals. 
     
     
       9. A computing system, comprising:
 one or more computers and one or more storage devices storing instructions that are operable, when executed by the one or more computers, to cause the one or more computers to perform a method for estimating a rotor speed of an alternating current (AC) machine, the method comprising: 
 determining a stator flux signal based on signals of voltage and current inputs to the AC machine; 
 determining a rotor flux signal of the AC machine based on the determined stator flux signal; 
 determining an electrical angle signal based on the determined rotor flux signal; 
 deriving, with a processing unit, an electrical frequency signal from the determined electrical angle signal; 
 sampling, with the processing unit, the derived electrical frequency signal at a predetermined sampling rate, and storing a predetermined number of electrical frequency sample values; 
 a median filter configured to generate a median value of a sequence of the predetermined number of electrical frequency sample values by eliminating spikes in the electrical frequency sample values without altering step changes, wherein the median filter is configured to eliminate spikes in an angular derivative signal when the electrical angle signal is reset each cycle; 
 evaluating, with the processing unit, the median value of the predetermined of electrical frequency sample values; 
 constantly monitoring stator current commands to the AC machine to determine a slip frequency value of the AC machine; 
 outputting, with the processing unit, the rotor speed of the AC machine by subtracting the determined slip frequency value from the evaluated electrical frequency median value; and 
 driving the AC machine based on the outputted rotor speed. 
 
     
     
       10. The computing system of  claim 9 , wherein determining a rotor flux signal of the AC machine based on the determined stator flux signal comprises:
 evaluating a difference between a result of a product of a stator resistance value with one of the current input signals and one of the voltage input signals to produce a difference signal; 
 performing an integration of the produced difference signal, wherein the integration involves a discrete integrator having a sampling rate and a time delay; 
 filtering using a high-pass filtering unit the integrated difference signal in order to minimize a potential accumulated offset value of the integrated difference signal; 
 filtering the high-pass filtered difference using a low-pass filtering unit to extract a residual offset value from the high-pass filtered difference signal; 
 reducing the high-pass filtered difference signal by subtracting the extracted residual offset value from the high-pass filtered difference signal; and 
 determining the rotor flux signal based on the reduced high-pass filtered difference signal using a rotor flux derivation unit. 
 
     
     
       11. The computing system of  claim 9 , wherein evaluating the median value of the predetermined set of electrical frequency sample values comprises:
 removing data spikes that are generated by a periodic resetting of the electrical angle signal when varying between −π radians and π radians. 
 
     
     
       12. The computing system of  claim 9 , wherein evaluating the median value of the predetermined set of electrical frequency sample values comprises:
 using the median filter involves a moving window of N digital samples and corresponding discrete delays. 
 
     
     
       13. The computing system of  claim 12 , wherein an additional low pass filtering unit is configured to filter output signals of the median filter in order to reduce noise signals, such as ripple noise signals. 
     
     
       14. The computing system of  claim 12 , wherein signals of voltage and current inputs to the AC machine are provided by a three-phase inverting unit, which is in turn coupled to a current controlling unit that monitors the signals of current inputs to the AC machine. 
     
     
       15. The computing system of  claim 12 , wherein inputs to the three-phase inverter are determined based on the electrical angle signal and command input signals provided via a field-oriented control unit. 
     
     
       16. The computing system of  claim 15 , wherein the command input signals include rotor speed and flux command signals. 
     
     
       17. A system for estimating a rotor speed of an alternating current (AC) machine, comprising:
 a coordinate transform unit configured to transform voltage and current signals into equivalent q-axis and d-axis stator current and voltage signals; 
 a rotor-flux estimating unit configured to derive a rotor flux signal based on the equivalent q-axis and d-axis stator current and voltage signals; 
 a median filter configured to generate a median value of a sequence of the predetermined number of electrical frequency sample values by eliminating spikes in the electrical frequency sample values without altering step changes, wherein the median filter is configured to eliminate spikes in an angular derivative signal when the electrical angle signal is reset each cycle; 
 an estimating unit configured to determine, with a processing unit, an electrical angle signal based on the determined rotor flux signal, to derive an electrical frequency signal from the determined electrical angle signal, to sample the derived electrical frequency signal at a predetermined sampling rate, and storing a predetermined number of electrical frequency sample values, to evaluating the median value of the predetermined number of electrical frequency sample values, to constantly monitor stator current commands to the AC machine to determine a slip frequency value of the AC machine, to output the rotor speed of the AC machine by subtracting the determined slip frequency value from the evaluated electrical frequency median value; and to drive the AC machine based on the outputted rotor speed. 
 
     
     
       18. The system of  claim 17 , further comprising:
 a high-pass filtering unit for filtering an integrated difference signal in order to minimize a potential accumulated offset value of the integrated difference signal, wherein the integrated difference signal is obtained by performing an integration of an evaluated difference between a result of a product of a stator resistance value with one of the current input signals and one of the voltage input signals to produce a difference signal; 
 a low-pass filtering unit for filtering the high-pass filtered difference to extract a residual offset value from the high-pass filtered difference signal; 
 a rotor flux derivation unit for determining the rotor flux signal based on the high-pass filtered difference signal that is reduced by the extracted residual offset value. 
 
     
     
       19. The system of  claim 17 , further comprising:
 a three-phase inverting unit that communicates signals of voltage and current inputs to the AC machine. 
 
     
     
       20. The system of  claim 19 , further comprising:
 a current controlling unit that monitors the signals of current inputs to the AC machine. 
 
     
     
       21. The system of  claim 20 , further comprising:
 a command signal unit that provides speed and flux command signals to a field-oriented control unit, which in turn provides current command signals to the three-phase inverting unit via the current controlling unit. 
 
     
     
       22. The computer-implemented method of  claim 1 , wherein the electrical angle signal is based on a rotor flux angle.